a) Field of the Invention
The present invention is directed to a protective layer or a protective coating for workpieces and to workpieces provided with such a protective coating, as well as to a process for producing a protective coating cn workpieces, wherein the workpieces in particular are made from carbon composite materials which are provided for use in a temperature range of approximately 500.degree. C. to more than 1000.degree. C., e.g., up to about 1600.degree. C.
b) Description of the Related Art
Protective layers for improving oxidation stab lity, particularly of carbon-containing materials, are already known in the prior art as will be made clear by several examples hereinafter.
For a protective layer for graphite- and carbon-containing materials, including substrates with a reinforced carbon-carbon structure, at increased temperatures, U.S. Pat. No. 4,465,777 discloses a composition containing a solids particle mixture with 50 to 70 percent by weight silicon carbide, 25 to 40 percent by weight silicon, 1 to 15 percent by weight boron and a minimum amount of magnesium oxide. This minimum magnesium oxide content serves to facilitate layer formation. According to this process, heating of the substrate surrounded by the mixture to form a coating thereon in an inert atmosphere in a temperature range of 3075.degree. F. (roughly 1698.degree. C.) to 3350.degree. F. (roughly 1851 .degree. C.) is claimed as sufficient.
An oxidation-resistant film for carbon-containing products, especially for crucible materials and casting-mold materials, is described in U.S. Pat. No. 4,559,270. The layer contains 20 to 70 percent by weight of mullite, 5 to 14 percent by weight of silica (SiO.sub.2), 5 to 70 percent by weight of SiC and/or B.sub.4 C and 5 to 14 percent by weight of a binder containing alkali silicate, ethyl silicate or colloidal SiO.sub.2. Loading temperatures of 1350.degree. C. are specified.
U.S. Pat. No. 4,535,035 describes an oxidation-resistant film for carbon-containing materials based on a mixture of nickel powder and silicon powder. The slurried mixture is sprayed onto a carbon-containing substrate and infiltrated into the pores of the substrate material by vacuum sintering in the range of 1200.degree. C. to 1400.degree. C.
A protective layer comprising a plurality of layers for carbon-containing materials is described in U.S. Pat. No. 4,471,023. The protective layer claimed therein is meant to be effective especially against cyclical thermal loading on carbon-containing substrates coated with a cover film of SiC. The base layer of the protective coating known from this reference is made from tetraethyl orthosilicate (TEOS) and serves for infiltration of the substrate. Another layer is formed of SiC particles in a liquid carrier system of alkali silicate; this layer is followed by a cover layer of alkali silicate without SiC particles. Finally, after application and drying of the various layers, a heat treatment is carried out at approximately 1800.degree. F. (approximately 994.degree. C.) in an inert atmosphere.
Finally, DE 40 34 001 A1 describes a coating for protecting against high-temperature oxidation of carbon-containing structural component parts. This coating is made from a powder mixture of MoSi.sub.2 and a material component with a low melting point with respect to the latter such as an oxide glass, a nonoxide glass, glass-ceramics or a combination thereof. Roughly 10 to 50 percent by weight of the mixture consists of MoSi.sub.2 particles which form a network which is fixedly embedded in the low-melting-point phase. Production of the protective layer is concluded by heating the workpiece coated with the powder mixture to 1300.degree. C. to 1700.degree. C. in air.